Jumping on the musical leaves, you can collect Munny. 1 through all of them time should net you about 500 Munnies. Go to the save point, go back to the world, and keep repeating this.

Another method is to have Sora and Riku go to The World That Never Was.

Have them complete the special portal, then drop to the other, each portal will net you a high level Fantasy that can sell for up to 1000 gil each. TWTNW only has three possible spawn points for the portal, making it easy to farm them repeatedly.

Since you are going aganst "Ventus" he will have quick, fast attacks. Use Flowmotion on any side of the area to dodge easily. And do a Kick Dive when he is barraging you with attacks since he cannot block it.